U.S. patent application number 14/689023 was filed with the patent office on 2015-10-22 for modular dc-dc converter.
The applicant listed for this patent is The Regents of the University of Colorado. Invention is credited to Hua Chen, Robert Warren Erickson, JR., Tadakazu Harada, Dragan Maksimovic.
Application Number | 20150303815 14/689023 |
Document ID | / |
Family ID | 54322833 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150303815 |
Kind Code |
A1 |
Chen; Hua ; et al. |
October 22, 2015 |
MODULAR DC-DC CONVERTER
Abstract
A modular dc-dc boost converter system is provided that can
substantially improve efficiency over a wide range of input and
output voltages. The system includes three modules: a buck module,
a boost module, and a dc transformer module. These modules are
interconnected such that the system output voltage is equal to the
sum of the output voltages of adc-dc converter module and a dc
transformer module. Depending on the operating point, one or more
modules may operate in passthrough mode, leading to substantially
reduced ac losses. The required capacitor size and the transistor
voltage ratings are also substantially reduced, relative to a
conventional single dc-dc boost converter operating at the same
input and output voltages.
Inventors: |
Chen; Hua; (Boulder, CO)
; Erickson, JR.; Robert Warren; (Boulder, CO) ;
Maksimovic; Dragan; (Boulder, CO) ; Harada;
Tadakazu; (Boulder, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Regents of the University of Colorado |
Boulder |
CO |
US |
|
|
Family ID: |
54322833 |
Appl. No.: |
14/689023 |
Filed: |
April 16, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61980419 |
Apr 16, 2014 |
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Current U.S.
Class: |
363/21.04 |
Current CPC
Class: |
H02M 3/1582 20130101;
Y02B 70/1491 20130101; H02M 2001/0054 20130101; Y02B 70/10
20130101; H02M 3/158 20130101; H02M 2001/0077 20130101 |
International
Class: |
H02M 3/335 20060101
H02M003/335 |
Claims
1. A modular dc-dc converter system, comprising: a dc-dc converter
module including a converter module input port and a converter
module output port; a dc transformer module including a transformer
module input port and a transformer module output port; a
controller that supplies one or more duty cycle signals to at least
one switch of the dc-dc converter module and one or more control
signals to the dc transformer module; a system input voltage port;
and a system output voltage port, wherein the converter module
input port of the dc-dc converter module is coupled to the system
input voltage port, the input port of the dc transformer module is
coupled to the system input voltage port, the output port of the
dc-dc converter module is coupled in series with the output port of
the dc transformer module and the series-coupled ports are coupled
to the system output voltage port.
2. The apparatus of claim 1 wherein the controller commands the
dc-dc converter to operate in a pass-through mode over a range of
input and output voltages.
3. The apparatus of claim 1 wherein the controller comprises a
microcontroller.
4. The apparatus of claim 1 wherein the transformer output port of
the dc transformer module and the converter module output port of
the dc-dc converter module share voltage stresses of the modular
dc-dc converter system.
5. The apparatus of claim 1 wherein the dc transformer module
comprises a zero-voltage switching dc-dc converter circuit.
6. The apparatus of claim 1 wherein the dc-dc converter module
comprises a hard-switched dc-dc converter module.
7. The apparatus of claim 1 wherein a second dc-dc converter module
is coupled in series between the transformer module input port and
the system input voltage port.
8. The apparatus of claim 7 wherein a second converter module input
port of the second dc-dc converter module is coupled in parallel
with the converter module input port of the dc-dc converter module
and a second converter module output port of the second dc-dc
converter module is coupled in series with the transformer module
input port.
9. A modular dc-dc converter system, comprising: a first dc-dc
converter module, including a first converter module input port and
a first converter module output port; a second dc-dc converter
module, including a second converter module input port and a second
converter module output port; a dc transformer module, including a
transformer module input port and a transformer module output port;
a controller that supplies at least one first duty cycle signal to
at least one switch of the first dc-dc converter module, at least
one second duty cycle signal to at least one switch of the second
dc-dc converter module, and one or more control signals to the dc
transformer module; a system input voltage port; and a system
output voltage port, wherein the first converter module input port
of the first dc-dc converter module and the second converter module
input port of the second dc-dc converter module are coupled
connected in parallel, and both the first dc-dc converter module
and the second dc-dc converter module are coupled to the system
input voltage port; the first converter module output port of the
first dc-dc converter module is coupled to the transformer module
input port of the dc transformer module; the second converter
module output port of the second dc-dc converter module is coupled
in series with the output port of the dc transformer module, and
the series-coupled ports are coupled to the system output voltage
port.
10. The apparatus of claim 9 wherein the controller is configured
to command the first dc-dc converter module to operate in
pass-through mode over a range of input and output voltages.
11. The apparatus of claim 9 wherein the controller is configured
to command the second dc-dc converter module to operate in
pass-through mode over a range of input and output voltages
12. The apparatus of claim 9 wherein the controller comprises a
microcontroller.
13. The apparatus of claim 9 wherein the transformer output port of
the dc transformer module and the second converter module output
port of the second dc-dc converter module share voltage stresses of
the modular dc-dc converter system.
14. The apparatus of claim 9 wherein the dc transformer module
comprises a zero-voltage switching dc-dc converter circuit.
15. The apparatus of claim 14 wherein the dc transformer module
includes an inductive transformer.
16. The apparatus of claim 14 wherein the dc transformer module
comprises isolated input and output terminals.
17. The apparatus of claim 9 wherein each of the first dc-dc
converter module and the second dc-dc converter module comprises a
hard-switched dc-dc converter module.
18. The apparatus of claim 9 wherein each of the first dc-dc
converter module and the second dc-dc converter module comprise at
least one of a buck dc-dc converter module, a boost dc-dc converter
module and a non-inverting buck-boost dc-dc converter module.
19. A method of controlling a modular dc-dc converter system, the
method comprising: providing a modular dc-dc converter system
comprising: a dc-dc converter module including a converter input
port and a converter output port; a dc transformer module including
a transformer input port and a transformer output port; a
controller that supplies at least one duty cycle signal to at least
one switch of the dc-dc converter module and at least one control
signal to the dc transformer module; a system input voltage port;
and a system output voltage port, wherein the converter input port
of the dc-dc converter module and the transformer input port of the
dc transformer module are coupled to the system input voltage port,
the converter output port of the dc-dc converter module is coupled
in series with the transformer output port of the dc transformer
module and the series-coupled ports are coupled to the system
output voltage port; and in a first operational state, controlling
the dc-dc converter module to operate with pulse-width modulation
(PWM) and controlling the dc transformer module with a fixed
conversion ratio; and in a second operational state, performing an
operation comprising at least one of (i) controlling the dc-dc
converter module to operate in a pass-through mode and (ii)
controlling the dc transformer module to shut down and controlling
the dc-dc converter module to operate with PWM.
20. The method of claim 19 wherein the operation performed in the
second operational state comprises controlling the dc-dc converter
module to operate in a pass-through mode and, in a third
operational state, performing an operation of controlling the dc
transformer module to shut down and controlling the dc-dc converter
module to operate with PWM.
21. The method of claim 20 wherein, the dc transformer module is
controlled to shut down by turning on a plurality of dc transformer
module output switches.
22. A method of controlling a modular dc-dc converter system, the
method comprising: providing a modular dc-dc converter system
comprising: a first dc-dc converter module including a first
converter input port and a first converter output port; a second
dc-dc converter module including a second converter input port and
a second converter output port; a dc transformer module including a
transformer input port and a transformer output port; a controller
that supplies at least one first duty cycle signal to at least one
switch of the first converter module, at least one second duty
cycle signal to at least one switch of the second converter module,
and one or more control signals to the dc transformer module; a
system input voltage port; and a system output voltage port,
wherein the first converter input port of the first dc-dc converter
module and the second converter input port of the second dc-dc
converter module are coupled in parallel, both the first dc-dc
converter module and the second dc-dc converter module are coupled
to the system input voltage port; the first converter output port
of the first dc-dc converter module is coupled to the transformer
input port of the dc transformer module; the second converter
output port of the second dc-dc converter module is coupled in
series with the transformer output port of the dc transformer
module, and the series-coupled ports are coupled to the system
output voltage port; and in a first operational state, controlling
the dc transformer module to shut down with dc transformer module
output switches turned on and controlling the second dc-dc
converter module to operate with PWM; and in a second operational
state performing an operation comprising at least one of (i)
controlling the first dc-dc converter module to operate in a
pass-through mode, controlling the second dc-dc converter module to
operate with pulse-width modulation (PWM) and controlling the dc
transformer module with a fixed conversion ratio and (ii)
controlling the second dc-dc converter module to operate in a
pass-through mode and controlling the first dc-dc converter module
to operate with PWM and controlling the dc transformer module with
a fixed conversion ratio.
23. The method of claim 22 wherein, in a third operational state,
in a third operational state, controlling the first dc-dc converter
module and the second dc-dc converter module to operate with PWM,
wherein the output voltage of the second dc-dc converter module is
limited.
24. The method of claim 22 wherein, in a third operational state,
controlling the first and second dc-dc converter modules to operate
in a pass-through mode and controlling the dc transformer module
with a fixed conversion ratio.
Description
FIELD
[0001] The present disclosure relates to DC-DC power conversion
that incorporating a DC transformer and converts an input voltage
to an output voltage.
BACKGROUND
[0002] A modular dc-dc conversion system to boost a voltage is
disclosed in the following patent application: [0003] Robert
Erickson, "Integrated Photovoltaic Module" U.S. patent application
Ser. No. 13/318,589, May 10, 2010, which is incorporated by
reference herein in its entirety as if fully set forth herein. The
approach in this reference provides a non-inverting buck-boost
converter arranged in series with a unidirectional DC transformer
(DCX) module. The reference does not disclose a DCX module whose
output port is connected in series with the output of a converter
module, such as a boost module.
[0004] A publication that provides a detailed analysis of a DC
transformer circuit, such as the DCX circuit shown in FIG. 8,
including design information, is the following: [0005] D. Jones and
R. Erickson, "Analysis of Switching Circuits through Incorporation
of a Generalized Diode Reverse Recovery Model into State Plane
Analysis," IEEE Transactions on Circuits and Systems I, vol. 60,
no. 2, pp. 479-490, February 2013, which is incorporated by
reference herein in its entirety as if fully set forth herein.
[0006] A publication that describes a method for controlling buck
and boost converters using pass-through modes is the following:
[0007] D. Jones and R. Erickson, "A Nonlinear State Machine for
Dead Zone Avoidance and Mitigation in a Synchronous Noninverting
Buck-Boost Converter," IEEE Transactions on Power Electronics, vol.
28, no. 1, pp. 467-480, January 2013, which is incorporated by
reference herein in its entirety as if fully set forth herein.
[0008] A DC-DC boost converter increases a DC input voltage
V.sub.in to produce a DC output voltage V.sub.out=MV.sub.in, where
the conversion ratio M is greater than or equal to one. An example
of a well-known implementation of a boost converter 10 is
illustrated in FIG. 1. In this circuit 10, a controller circuit
drives a transistor gate 12 with a repetitive signal that causes
the transistor Q to be ON for a time DT.sub.s, and OFF for a time
(1-D)T.sub.s, where D is the transistor duty cycle and T.sub.s is
the switching period. When the transistor Q is ON, energy from an
input source is stored in the inductor L. When the transistor Q is
OFF, the diode D becomes forward-biased by an inductor current, and
energy stored in the inductor L is released to the output. To the
extent that the circuit elements have low power loss, the output
voltage is given by V.sub.out=V.sub.in/(1-D), and the efficiency
.eta.=P.sub.out/P.sub.in can approach 100%. A bi-directional
converter 20 that is an extension of the conventional DC-DC boost
converter is illustrated in FIG. 2, in which a pair of transistors
Q1 and Q2 and a pair of diodes D1 and D2 allow the inductor current
to be either positive or negative, so that power can flow from
either V.sub.in to V.sub.out or V.sub.out to V.sub.in.
[0009] It is well known that a variety of loss mechanisms reduce
the efficiency of the boost converters of FIG. 1 and FIG. 2. These
loss mechanisms can be broadly grouped into DC losses and AC
losses. In this disclosure, DC losses refer to losses that do not
depend directly on the switching frequency, such as losses arising
from the forward voltage drops of the semiconductor devices and
losses caused by the DC resistance of the inductor winding. AC
losses refer to losses that increase with switching frequency, such
as semiconductor switching losses caused by transistor and diode
switching times, diode reverse recovery, semiconductor output
capacitances, and transistor drive power. The inductor also
exhibits AC losses caused by core loss as well as AC winding losses
arising from the skin and proximity effects. As a result of these
loss mechanisms, the conventional boost converter circuit may
exhibit substantially degraded efficiency. Furthermore, the
efficiency is a function of input and output voltage, switching
frequency, and output power. FIG. 3 illustrates typical efficiency
curves of a boost converter, for several values of resistive load.
It can be seen that the efficiency degrades as the duty cycle (and
hence also output voltage) is increased.
[0010] Typically, power converters are thermally limited by their
cooling systems, and these cooling systems may have significant
size and cost. For a given cooling technology and cooling system
size, there is a fixed amount of loss that can be tolerated while
maintaining an acceptable temperature rise. In a thermally limited
system, improvement of efficiency means that the output power can
be increased. For example, if the efficiency can be increased from
96% to 98%, then the loss is approximately halved. Assuming that
the system is still thermally-limited and the cooling system size
is maintained constant, then the rated output power can be doubled
and the cost per watt of output power is halved. Ultimately, it is
desirable to increase the ratio P.sub.out/P.sub.loss so that the
converter cost per watt, or cooling system size and cost, are
decreased.
[0011] A conventional boost converter also exhibits reduced
efficiency at low output power, as a result of AC losses. Converter
efficiency over a range of output powers and voltages is
increasingly important because the converter may operate at partial
power for a substantial fraction of the time. For example, power
converters for solar power systems are characterized by a weighted
efficiency that accounts for efficiency not only at rated power,
but also at lower powers corresponding to less than full
irradiance. Power converters for electric vehicle applications must
operate over driving profiles having a wide variety of speeds and
accelerations, corresponding to a variety of converter output
voltages and powers; improvement of efficiency at all of these
operating points is needed to improve the effective miles per
gallon (MPGe) of the vehicle. Power converters for grid interface
of wind turbines must also operate efficiently with a wide range of
voltages and output powers, corresponding to a range of wind
speeds. FIG. 4 illustrates a typical efficiency curve for a
conventional boost converter, operating at a constant output
voltage and with variable output power. It is desirable to increase
the efficiency not just at maximum power, but also at lower
powers.
[0012] AC switching losses can be reduced by reduction of the
switching frequency. However, this necessitates use of larger
inductor and capacitor elements, which are more expensive. The
larger inductor may also exhibit higher DC resistance. Therefore,
it is often undesirable to reduce the switching frequency, and
solutions are needed that achieve high efficiency without
sacrificing switching frequency.
[0013] The size of the output capacitor is often limited by its
root-mean-square (RMS) current rating. The RMS capacitor current
increases as the duty cycle is increased. To reduce the size and
cost of this capacitor, an improved circuit is needed that can
boost the voltage substantially, while maintaining relatively low
RMS capacitor current.
[0014] High-voltage power semiconductor devices typically exhibit
increased switching times and increased switching losses. In a
boost converter system, for example, it may be desirable to avoid
use of high-voltage semiconductor devices, employing multiple
lower-voltage devices instead. A well-known example of this is a
multilevel converter; a three-level boost converter is illustrated
in FIG. 5. This converter circuit 30 can achieve some of the goals
delineated here, including reduction of AC losses and use of
semiconductors with reduced voltage rating. However, it operates
with substantially increased capacitor RMS currents, and hence
requires expensive capacitors.
SUMMARY
[0015] In various implementations, for example, a modular DC-DC
converter is provided that employs semiconductors with reduced
voltage ratings, while also reducing the capacitor RMS
currents.
[0016] In one implementation, DC-DC power conversion incorporating
a DC transformer that converts an input voltage to an output
voltage is provided. In various implementations, a modular DC-DC
power conversion is provided to improve converter efficiency over a
wide range of conversion ratios and output powers. In one
particular implementation, a modular architecture includes a DC
transformer (DCX) module and at least one converter module capable
of being operated in a pass-through mode. For example, a modular
architecture may include a DC transformer module and at least one
of a boost converter module, a buck converter module and a
non-inverting buck-boost converter module. The modules may be
configured as and controlled such that efficiency is improved.
[0017] In one implementation, a boost DC-DC converter improves the
efficiency of a DC-DC boost converter system, through reduction of
the AC losses; improves converter efficiency over a range of
operating points, i.e., a range of conversion ratios and output
powers; reduces capacitor size, through reduction of the RMS
capacitor current(s); and/or employs semiconductor power devices
having reduced voltage ratings and better performance.
[0018] In various implementations, a modular DC-DC boost converter
architecture employs partial-power modules performing DC
transformer (DCX), buck, and boost functions. These modules are
able to operate with ultra-high efficiency over a restricted range
of operating points, and are combined into a system architecture
that performs the required DC-DC boost conversion function. The DCX
module, for example, is able to perform an isolated boost function
at a fixed conversion ratio, with very high efficiency. Boost and
buck modules may operate with a restricted range of conversion
ratios where their efficiency is very high and where the capacitor
current stresses and inductor applied ac voltages are substantially
reduced. Voltage sharing between modules allows use of
lower-voltage semiconductor devices having better characteristics,
and also reduces AC losses. One or more controllers may command the
switching of the semiconductor devices of the modules. These
controller(s) may employ pass-through modes, in which one or more
modules simply connect their input and output ports to achieve a
conversion ratio of unity; this improves efficiency by eliminating
the AC loss of the module(s). The AC loss of the overall system is
reduced, and hence the efficiency is increased over a range of
output voltages and powers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] For a more complete understanding of the invention,
reference is made to the following description and accompanying
drawings, in which:
[0020] FIG. 1 is an example boost converter with unidirectional
power flow;
[0021] FIG. 2 is an example boost converter with bidirectional
power flow;
[0022] FIG. 3 is a typical plot of efficiency vs. duty cycle for a
prior art boost converter;
[0023] FIG. 4 is a typical plot of efficiency vs. output power, for
a fixed output voltage, for a prior art boost converter and also
for an example implementation of a boost converter provided
herein;
[0024] FIG. 5 is an example three-level boost converter;
[0025] FIGS. 6 A and 6B include block diagrams of an example
modular DC-DC converters;
[0026] FIG. 7 is an representation of an example DC transformer
module;
[0027] FIGS. 8 A and 8B and 8C include schematic diagrams of
example DC transformer modules;
[0028] FIG. 9 illustrates example DCX waveforms;
[0029] FIG. 10 is a representation of an example buck or boost
converter module having conversion ratio M(D);
[0030] FIG. 11 is a schematic of an example boost converter module
with bidirectional power flow;
[0031] FIG. 12 illustrates a high efficiency operating point having
a boost ratio of 3;
[0032] FIG. 13 is a map of various example modes of operation of
the DC-DC boost converter shown in FIG. 6B;
[0033] FIG. 14 illustrates example measured module waveforms for
operation in the DCX+Boost mode;
[0034] FIG. 15 illustrates an example implementation of control for
a modular boost dc-dc system using a microcontroller and sensor
signals;
[0035] FIG. 16 illustrates top-level blocks of an example
implementation of a modular boost dc-dc system controller;
[0036] FIG. 17 shows an example relationship of a controller
operating status to operating modes of a modular boost system;
[0037] FIG. 18 is a detailed block diagram of an example main
controller block, which implements average current control;
[0038] FIG. 19 is a detailed block diagram of an example DCX
voltage limit block;
[0039] FIG. 20 is a detailed block diagram of an example control
mixer block; and
[0040] FIG. 21 is a detailed block diagram of an example control
mixer block.
DETAILED DESCRIPTION
[0041] An example implementation of a modular DC-DC converter 100
architecture is illustrated in FIG. 6A. In this particular
implementation, a DC transformer module 102, DCX module 102, is
provided in a stacked configuration with a DC-DC converter module
104 capable of operating in a pass-through mode. The DC-DC
converter module 104 may, for example, include one or more of a
Buck DC-DC converter module, a Boost DC-DC converter module, a
non-inverting Buck-Boost DC-DC converter module or any other type
of converter module capable of being operated in a pass-through
mode.
[0042] The implementation of FIG. 6A includes the DCX module 102
and the DC-DC converter module 104 arranged in such that the input
ports 106, 108 of the DCX module 102 and the DC-DC converter module
104, respectively, are each coupled to an input voltage V.sub.in.
An output port 110 of the DCX module 102 is coupled in series with
an output port 112 of the DC-DC converter module 104. In this
manner, an output voltage of the modular DC-DC converter 100 is a
sum of the individual output voltages V.sub.DCXout of the DCX
module 102 and V.sub.DC-DCCout of the DC-DC converter 104.
[0043] Another example implementation of a modular DC-DC boost
converter architecture 120 is illustrated in FIG. 6B. With this
approach, several partial-power converter modules 122, 124 and 126
are combined such that the system efficiency is optimized over a
range of voltage and power operating points. By control of the
modules 122, 124 and 126, the efficiency over the given range can
be substantially higher than in a conventional full-power boost
approach. In addition, the modules can share the voltage stresses,
allowing lower-voltage semiconductor devices to be employed.
Additionally, the choice of module types affects the sizes of the
magnetic and capacitive elements of the system; the architecture
allows the capacitive elements to be much smaller than in competing
approaches such as the multilevel boost architecture of FIG. 5.
[0044] FIG. 6B illustrates another example modular dc-dc converter
system 120 including two DC-DC converter modules 122, 126 and a DC
transformer module 124. In this implementation, the first DC-DC
converter module 122 is a buck converter module 122, and the second
DC-DC converter module 126 is a boost converter module 126. This
particular arrangement of the implementation shown in FIG. 6B is,
however, merely an example implementation. The individual DC-DC
converter modules (buck 122 and boost 126) or DC transformer module
124 may be arranged in other orders or may include other types of
modules. Each of the DC-DC converter modules 122 and 126, for
example, may include at least one or a buck converter module, a
boost converter module, a non-inverting buck-boost converter module
or any other type of DC-DC converter module capable of operating in
a pass-through mode.
[0045] In the particular implementation shown in FIG. 6B, for
example, the first DC-DC converter module 126 and the DC
transformer module 124 are coupled in series along a first, upper
branch of the modular DC-DC converter 120 and the second DC-DC
converter module 126 is arranged along a second, lower branch of
the modular DC-DC converter 120. An input port 128, in this case
including two input terminals, of the first DC-DC converter module
122 is coupled in parallel with an input port 136 of the second
DC-DC converter module 126 and the input ports are coupled to an
input voltage Vin of the modular DC-DC converter 120. An output
port 130 of the first DC-DC converter module 122 is coupled to an
input port 132 of the DC transformer module 124. An output port 134
is coupled in series with an output port 138 of the second DC-DC
converter module 126. The output voltage Vout of the modular DC-DC
converter module 120, thus, is a sum of the output voltages of the
DC transformer module 124 and the second DC-DC converter module
126.
[0046] In one implementation of a DC Transformer (DCX) module 140
illustrated in FIG. 7 is a zero-voltage switching DC-DC converter
circuit containing a conventional (inductive) transformer. In this
implementation, this circuit is optimized to achieve a very high
efficiency at a single voltage conversion ratio
V.sub.DCX-out/V.sub.DCX-in=N.sub.DCX. The input and output
terminals are isolated. This type of circuit is generally not able
to control its voltage conversion ratio. One well-known DCX circuit
is the Dual Active Bridge converter 150, illustrated in FIG. 8.
This circuit 150 is capable of bidirectional power flow, and it
achieves zero-voltage switching of the power semiconductors on the
primary and secondary sides of a transformer 154 through addition
of a small tank inductor Ltank. The transformer physical turns
ratio is approximately the same as the conversion ratio N.sub.DCX
defined above. In an exemplary realization, the DCX module of FIG.
6B is realized using the dual-active bridge circuit of FIG. 8, with
typical waveforms as illustrated in FIG. 9. This schematic
illustrates use of power MOSFETs 152 in full-bridge configurations
on the primary and secondary sides of the transformer. These
MOSFETs 152 include built-in body diodes, and are driven by gate
driver circuits 154. A DCX controller 158 produces control signals
that command the gate drivers to turn the MOSFETs 152 on and off.
Each MOSFET 152 conducts with a duty cycle of approximately 50%,
except for a small dead time inserted by the DCX controller to
ensure that the series-connected upper and lower MOSFETs 152 do not
simultaneously conduct. FIG. 9 shows typical measured waveforms:
the two upper traces are gate drive signals for a pair of
series-connected upper and lower MOSFETs 152. Each MOSFET 152
conducts when its gate drive signal is high, and the dead time
where both gate drive signals are low can be seen in the figure.
Also shown in FIG. 9 (middle trace) is an example transformer
primary winding current waveform. At a selected value of output
current, this transformer winding current waveform is nearly
trapezoidal, with peak value only slightly greater than the dc
current. The lower trace in this example is the approximately 50%
duty cycle voltage waveform observed at the switch node between the
series-connected upper and lower MOSFETs. The voltage waveforms at
the other three switch nodes are similar. For this discussion, it
is assumed that the DCX module operates with a conversion ratio
that is fixed and equal to N.sub.DCX. Alternatively, the controller
can turn the DCX module off, with the input and output terminals
operated as short circuits or as open circuits.
[0047] Other example implementations of DC transformer (DCX)
transformer modules 160, 170 are illustrated in FIGS. 8B and 8C,
respectively. In these implementations, the DCX modules are
uni-directional DCX modules in which either the input-side or
output-side switches are replaced with diodes D1 through D4 as
shown in FIGS. 8B and 8C, respectively. In FIG. 8B, for example,
the output-side switches Q5 through Q8 of the dual active bridge
converter implementation shown in FIG. 8A are replaced with diodes
D1 through D4 and allow the DCX module of FIG. 8B to provide power
flow from an input voltage V.sub.in side to an output voltage
V.sub.out side of the DCX module. In the implementation of FIG. 8C,
however, switches Q1 through Q4 of the dual active bridge converter
implementation shown in FIG. 8A are replaced with diodes D1 through
D4. In this implementation, the DCX module provides power flow from
an output voltage V.sub.out side to an input voltage V.sub.in
side.
[0048] An example Buck or Boost PWM module 180 illustrated in FIG.
10 is a conventional hard-switched DC-DC converter. A schematic of
an example boost converter module 190 is given in FIG. 11. In this
example, semiconductor switches are realized using power MOSFET
transistors Qt, Q2 having fast recovery body diodes, and power is
able to flow in either direction. A controller 192 generates
pulse-width modulated (PWM) gate drive signals to control the
voltage conversion ratio M.sub.boost(D)=V.sub.out/V.sub.in=1/(1-D)
of this module, where D is the duty cycle of the lower MOSFET Q2.
The upper MOSFET Q1 is driven with the complement of the gate drive
signal applied to the lower MOSFET Q2, with the exception of a
small deadtime inserted into the gate drive signals to ensure that
the two MOSFETs Q1, Q2 do not simultaneously conduct. In this
implementation, the buck module is identical to the boost module,
but with the input and output terminals interchanged.
[0049] A buck or boost module achieves maximum efficiency in
pass-through mode, where the conversion ratio is M(D)=1. This is
achieved by causing the high-side semiconductor switch to remain in
the on state: D=1 for the buck converter, or D=0 for the boost
converter. There is no switching loss in the passthrough mode, and
the input is connected to the output through the high-side
semiconductor device and the inductor. Very high efficiencies are
achieved in passthrough mode. Neighboring operating points, with
pulse-width modulation at a duty cycle D near the passthrough
value, also achieves high efficiency but with some switching loss.
Operation at PWM duty cycles farther from the passthrough value is
undesirable because of the increased ac losses in the magnetics,
increased ac capacitor currents, and overall reduced
efficiency.
[0050] To appreciate how the modular boost architecture can improve
the efficiency at high output bus voltages, consider the operating
point illustrated in FIG. 12. For this example, an overall system
conversion ratio is V.sub.out/V.sub.in=3. If a single boost
converter were employed in this example, it would operate with a
high duty cycle of over 0.7, and would exhibit relatively low
efficiency. However, in the modular boost configuration 200, with
N.sub.DCX=2, the buck and boost modules operate in passthrough mode
with very high efficiency. The DCX output voltage of
N.sub.DCXV.sub.in is added to the boost output voltage of V.sub.in,
leading to a total dc output voltage of
(1+N.sub.DCX)V.sub.in=3V.sub.in. The system efficiency is very high
at this operating point because the buck and boost modules operate
in pass-through mode, and the DCX operates with very high
efficiency at its optimized conversion ratio of 2. The dc output
voltage can be increased by increasing the boost duty cycle, and
the dc output voltage can be decreased by decreasing the buck duty
cycle.
[0051] FIG. 13 summarizes how the modules may be controlled. In
this example, it is assumed that the input voltage can vary over
the range V.sub.in,min.ltoreq.V.sub.in.ltoreq.V.sub.in,max, and the
output voltage is controlled over the range
V.sub.in.ltoreq.V.sub.out.ltoreq.V.sub.out,max. A maximum
conversion ratio M.sub.max is also assumed. Additionally, the
operation modes of FIG. 13 assume that the semiconductor device
voltages are constrained to be no greater than V.sub.Q,max and this
limit is taken to be the maximum allowed voltage at the input or
output port of any module. FIG. 13 illustrates four example system
operating modes. In the region labeled "DCX+Boost", the buck
converter operates in pass-through mode while the boost converter
operates with PWM at D>0. The DCX operates, with fixed
conversion ratio N.sub.DCX. In the region labeled "DCX+Buck", the
boost converter operates in pass-through mode, while the buck
converter operates with PWM at D<1. In the region labeled "DCX
Buck Boost", both the buck and the boost converters operate with
PWM away from pass-through mode: the buck converter limits its
output voltage to approximately V.sub.Q,max/N.sub.DCX, so that the
voltage at the DCX output does not exceed V.sub.Q,max. The boost
converter produces a voltage of (V.sub.out-V.sub.Q,max). Finally,
for the region labeled "Boost only", the DCX is shut down, with its
output-side switches turned on so that the DCX output voltage is
zero. The boost converter operates with PWM to produce the required
output voltage. Control to implement this strategy is described in
a later section of this disclosure.
[0052] Thus, in various boost implementations, a modular boost
architecture allows substantial improvement of system efficiency
over a range of operating points that require substantial boosting
of the voltage. The system architecture causes the output dc
voltage to be shared between the series-connected DCX and boost
modules, reducing the voltage applied to any individual
semiconductor switch. Hence, the modules can employ
lower-voltage-rated devices having better characteristics. Because
of the reduced module voltage and parallel input current paths, the
individual modules are rated at a fraction of the total system
output power.
Example Controller Architecture
[0053] A possible control objective in the modular boost dc-dc
converter system is to regulate the dc output voltage V.sub.out at
a reference value set within a controller. In this implementation,
the dc output voltage is to be regulated against static and dynamic
variations in input voltage V.sub.in and the output load current.
In a conventional single dc-dc converter system, the voltage
regulation feedback loop is relatively simple: the output voltage
is sensed and compared to a reference; the error between the sensed
bus voltage and the reference is processed by a controller to
determine the converter control signal, i.e., the duty cycle of the
converter power switches. In a modular dc-dc boost system, the
control loop is more complex since power is processed by two, three
or more interconnected converter modules. In the implementation of
FIG. 6B, for example, one or more controllers control a controller
a boost DC-DC converter module, a buck DC-DC converter module, and
a DCX module. In one implementation, such as the converter shown in
FIG. 6B for example, control signals for the three modules are as
follows: [0054] Buck module: buck converter PWM signal with duty
cycle d.sub.buck, 0.ltoreq.d.sub.buck.ltoreq.1, where d.sub.buck=0
in the shut-down mode, and d.sub.buck=1 in the pass-through mode;
[0055] Boost module: boost converter PWM signal with duty cycle
d.sub.boost, 0.ltoreq.d.sub.boost<1, where d.sub.boost=0 in the
boost pass-through mode; [0056] DCX module: DCX enable signal that
indicates if the DCX module is active or shut down. The modular
dc-dc boost controller determines and coordinates the module
control signals in response to the voltage error signal, as well as
in response to the location of the system operating point in the
space of input and output voltages. Determination and coordination
of the module control signals can be accomplished in a number of
ways. This section summarizes a centralized modular dc-dc boost
system controller 210 architecture, having the high-level structure
illustrated in FIG. 15. Sensed signals (module voltages and
currents) are received by a single central controller, in this
example, implemented in a microcontroller chip. This
microcontroller includes analog-to-digital converters (ADCs) that
receive the sensed signals, digital implementation of control
algorithms, and PWM outputs that provide the PWM signals listed
above: the buck converter PWM signals, boost converter PWM signals,
and the DCX module PWM signals including primary and secondary PWM
signals that are generated based on the DCX enable signal.
[0057] FIG. 16 shows a block diagram of one implementation of a
controller architecture 220 that illustrates how the module control
signals are generated based on the sensed power-stage signals and
the output voltage reference v.sub.out,ref. The "voltage/current
limiter" block limits adjusts the output voltage reference value so
that it always stays within the expected range. Furthermore, the
output voltage reference can be adjusted if a sensed steady-state
inductor current is too high, thus providing an over-current
protection function. Based on the sensed output voltage, the
reference output voltage, and the sensed inductor currents, the
"main controller" determines a single control variable called
d.sub.control. The main controller includes a dynamic response
designed to ensure stability and well-behaved transient responses
of the closed-loop controlled system. By design, the main control
variable d.sub.control is between 0 and 2.
[0058] The purpose of the "DCX voltage limiter" is to ensure that
the DCX output voltage stays below a set value V.sub.Q,max. The
output of the DCX voltage limiter is an auxiliary control variable
d.sub.limit, which is between 0 and 1.
[0059] The "boost only" block decides if the buck module should be
turned on or not depending on the requested bus voltage. The "boost
only" block outputs an auxiliary control variable
d.sub.bk.sub.--.sub.shn, which is between 0 and 1.
[0060] The "control mixer" takes the main control variable
d.sub.control, and the two auxiliary control variables, d.sub.limit
and d.sub.bk.sub.--.sub.shn, and determines the module control
signals d.sub.buck (buck module duty cycle), d.sub.boost(boost
module duty cycle) and DCX_en (DCX enable).
[0061] FIG. 17 shows how the controller operating status is related
to the modular boost system operating mode. In the Boost mode, the
"boost only" block shuts down the buck and the DCX modules, and the
voltage regulation loop is closed through the main controller. In
the DCX+Buck+Boost mode, the "DCX voltage limiter" block regulates
the DCX output to V.sub.Q,max, and the "main controller" regulates
the output voltage. In the DCX+Buck or DCX+Boost modes, the "main
controller" regulates the output voltage. The value of the main
control variable d.sub.control decides if the system is in the
DCX+boost mode (1<d.sub.control<2), or in the DCX+buck mode
(0<d.sub.control<1).
[0062] A more detailed block diagram of a "main controller" 230 is
shown in FIG. 18. This is a standard average current mode control
with an outer voltage loop including the voltage-loop compensator
G.sub.cv and an inner current loop including the current-loop
compensator G.sub.ci. The inner current loop is setup to regulate
the average of the buck and the boost inductor currents. The
compensators G.sub.ci and G.sub.ev are simple
proportional-plus-integral (PI) compensators. The range of
i.sub.ref is limited so that the dynamic range of the average
inductor current is limited. The main control variable
d.sub.control is by design limited between 0 and 2.
[0063] A more detailed block diagram of a "DCX voltage limit" block
240 is shown in FIG. 19. The DCX output voltage v.sub.Dcx is
obtained by subtracting the sensed boost output voltage from the
sensed output bus voltage. v.sub.Dcx is compared to a reference
value of V.sub.Q,max, denoted V.sub.Q,max-ref, and the error is
processed by G.sub.climit, which is a
proportional-plus-integral-plus-derivative (PID) compensator
producing the auxiliary control variable d.sub.limit, which is
between 0 and 1. If the DCX output voltage is less than
V.sub.Q,max-ref, the auxiliary control variable d.sub.limit
saturates to 1. If the DCX output exceeds 400 V (which may occur in
the DCX+Buck+Boost mode), the auxiliary control variable
d.sub.limit becomes less than 1.
[0064] A "boost only" block is implemented as an up/down counter.
When the requested output voltage reference is greater than
V.sub.Q,max-ref, it counts down from 1 to 0; when the requested
output voltage is less than V.sub.Q,max-ref, it counts up from 0 to
1.
[0065] A detailed design of a "control mixer" block 250 is shown in
FIG. 20. The buck module control variable (duty cycle d.sub.buck)
is obtained by:
[0066] 1. Taking the minimum of d.sub.control and d.sub.limit;
[0067] 2. Subtracting d.sub.bk.sub.--.sub.shn to get
d'.sub.buck;
[0068] 3. Limiting the final d.sub.buck between 0 and 1
If the DCX output is greater than V.sub.Q,max-ref, then d.sub.limit
drops to a value smaller than d.sub.control, so that d.sub.buck is
determined by d.sub.limit. In this case, the buck module is
effectively controlled to limit the DCX output voltage to
V.sub.Q,max-ref. If the DCX output voltage is less than
V.sub.Q,max-ref, then d.sub.limit=1. In this case, if
d.sub.control<1, d.sub.buck is determined by d.sub.control,
which means that the buck module regulates the system output
voltage. This occurs in the DCX+Buck mode. If d.sub.control>1,
then d.sub.buck=1, and the buck module is in the pass-through mode.
This occurs in the DCX+Boost mode. If the requested bus voltage is
less than V.sub.Q,max-ref, then d.sub.bk.sub.--.sub.shn, gradually
counts from 0 to 1, which ultimately leads to d'.sub.buck.ltoreq.0,
and the resulting d.sub.bk=0 shuts down the buck module. This
occurs in the Boost mode.
[0069] The DCX control signal DCX.sub.en shuts down the DCX module
whenever d'.sub.buck.ltoreq.0. The boost module control variable
d.sub.boost obtained by passing d.sub.control-1, through a 0 to 1
limiter. When d.sub.control<1, then d.sub.boost=0, which means
that the boost module in the pass-through mode. This occurs in the
system DCX+buck model. Otherwise the boost module regulates the bus
voltage.
[0070] With the control approach described above, all mode-changing
decisions are based on values of the main and the auxiliary control
variables, and all mode transitions are smooth and occur according
to the internal state of each controller block. However, with the
"control mixer" design in FIG. 20, one potential problem is that
when the buck or the boost module enters its pass-through or
shut-down mode, the module is no longer controlled unless the input
or the output voltage changes. As a result, upon transition to
pass-through or shut-down mode the inductor current may ring at the
power stage natural frequency. To address this issue, the sensed
buck and the boost inductor currents can be added to the "control
mixer" as shown in in FIG. 21. The band-pass filters G.sub.BPF pass
only the components of the sensed current around the natural
frequency of the power stage. In response, the controller will
respond to and suppress a ringing disturbance in the corresponding
inductor current.
[0071] Many implementations have been described with reference to
the accompanying Figures. Various features introduced in particular
implementations are also intended to be used in other
implementations where appropriate. For example, while one or more
features may be described with respect to one or more particular
implementations, this is merely for convenience and is not intended
to imply that those features are only contemplated to be used in
that particular implementation.
[0072] Although many implementations have been described above with
a certain degree of particularity, those skilled in the art could
make numerous alterations to the disclosed embodiments without
departing from the spirit or scope of this invention. The number
and type of modules (e.g., DC-DC converter modules, DC transformer
modules) in various implementations of a modular DC-DC converter
could be varied as well as their arrangement within the modular
DC-DC converter. For example, although the implementation of FIG.
6B describes a boost modular DC-DC converter, the implementation is
not limited to just boos configurations. Any other DC-DC converter
module capable of operating in a pass-through mode may be used
instead of another particular DC-DC converter module. Similarly,
various DC transformer modules may be used instead of the
particular DC transformer modules described herein.
[0073] All directional references (e.g., upper, lower, upward,
downward, left, right, leftward, rightward, top, bottom, above,
below, vertical, horizontal, clockwise, and counterclockwise) are
only used for identification purposes to aid the readeras
understanding of the present invention, and do not create
limitations, particularly as to the position, orientation, or use
of the invention. Joinder references (e.g., attached, coupled,
connected, and the like) are to be construed broadly and may
include intermediate members between a connection of elements and
relative movement between elements. As such, joinder references do
not necessarily infer that two elements are directly connected and
in fixed relation to each other. It is intended that all matter
contained in the above description or shown in the accompanying
drawings shall be interpreted as illustrative only and not
limiting. Changes in detail or structure may be made without
departing from the spirit of the invention as defined in the
appended claims
[0074] It is also to be understood that the following claims are
intended to cover all of the generic and specific features of the
invention herein described and all statements of the scope of the
invention which, as a matter of language, might be said to fall
therebetween.
* * * * *